The network has eight internal papers showing the results of tests on the European Pressurised Reactor (EPR) that, it claims, reveal defects in the mechanism that controls the nuclear reaction. These defects, it says, could cause an explosion sending a massive cloud of radiation into the atmosphere.

The documents, leaked by an insider at the French electricity firm EDF, which will run the new Flamanville 3 power station, date from between 2004 and 2009. "They show the main arguments in favour of the EPR ... are false," a spokesperson for Sortir du Nucléaire said.

The news will cause consternation among campaigners against nuclear power in the UK as EDF is one of several companies hoping to build the next generation of British reactors over the next seven years.

Philippe Brousse, Sortir du Nucléaire's director, said the documents revealed that in certain circumstances, the reactor ejected the cluster rods that regulate the nuclear reaction, causing a leak of coolant and, eventually, an explosion.

A committee of experts asked by Sortir du Nucléaire to examine the documents concluded: "It seems that the conception of the EPR increases the risk of a Chernobyl-type accident, with the destruction of the reactor chamber and the massive dispersion of nuclear radiation into the atmosphere."

Sortir du Nucléaire claims EDF kept the findings secret and accused it of taking risks "for economic reasons". However, EDF said the documents were part of the "normal process of analysing risks" applied to all nuclear reactors.

"No conclusion can be drawn from them at the moment," said a spokesperson, adding that the papers form part of a report to go before France's nuclear safety authority by the end of the year.

Flamanville 3 is the first of the new EPRs developed by France's nuclear company Areva using French and German technology to be built in France.

• This article was amended on 17 March 2010 to make clear that Flamanville 3 is the first of the new EPRs to be built in France, rather than worldwide.